p27KIP1 is a tumor suppressor gene in mice and a negative prognostic indicator in human cancers. Since the p27 gene is rarely mutated, there is the unique possibility to restore the function of a tumor suppressor by inhibiting negative regulation. We developed a functional genomics assay to discover regulators of p27. Inhibitor of DNA binding 3 (Id3) and galactokinase (Glk1) were isolated. The identification of ld3, a known cell cycle regulator whose expression inversely correlates with p27, validates our approach. Glk1 is a novel regulator of p27, and preliminary results indicate that Glk1 phosphorylates p27. The objectives of this proposal are to ascertain the mechanisms used by Id3 and Glk1 to decrease p27 protein. Since Id3 represses gene expression, Id3 inhibition of p27 transcription will be tested. For Glk1, the phosphorylation site in p27 will be identified, and the biological significance of this event determined. Additionally, tumor studies in mice carrying mutations for the Glk1 target site and mice overexpressing Glk1 will be conducted. [unreadable] [unreadable]